While there are numerous metal oxide sintered bodies, some are known to be more consolidated as they are more light transmissive. In particular, those sintered bodies having undergone hot isostatic pressing (HIP) step in the sintered body manufacturing process are confirmed to develop outstanding light transmittance. Since light-transmissive metal oxide sintered bodies are recently used in a variety of optical applications, active efforts are widely made on their development.
For example, JP-B H02-002824 (Patent Document 1) discloses a method comprising the steps of firing a ceramic compact (PLZT) composed mainly of lead, lanthanum, zirconium, and titanium oxides in vacuum to a density which is at least 97% of the theory density, burying the sintered body in a heat-resistant container which is closely packed with a powder of at least one type selected from fused aluminum oxide, fused zirconium oxide, and fused magnesium oxide, the powder having a particle size of 50 to 3,000 μm, and effecting HIP treatment. By this method, allegedly, ceramics having very high transparency and denseness for optical devices are consistently manufactured in a mass scale.
Also, JP-B H02-025864 (Patent Document 2) discloses a method for manufacturing a light-transmissive zirconia sintered body comprising the steps of firing a shaped body composed of at least 2 mol % of Y2O3, 3 to 20 mol % of TiO2, and ZrO2 in an oxygen-containing atmosphere, HIP treatment, and oxidative treatment. By this method, allegedly, light-transmissive zirconia sintered bodies having improved light transmission and high refractive index are manufactured.
Further, JP-A H03-275560 (Patent Document 3) discloses a method for manufacturing a light-transmissive yttrium-aluminum-garnet sintered body having a linear transmittance of at least 75% of infrared light of wavelength 3 to 4 μm at a thickness of 3 mm, comprising the steps of compacting powder, sintering to a high density, and HIP treatment at 1,500 to 1,800° C. and 500 kg/cm2 or higher; and JP-A H03-275561 (Patent Document 4) discloses a method for manufacturing a light-transmissive YAG sintered body comprising the steps of hot pressing a YAG powder having a purity of at least 99.6% and a specific surface area (BET value) of at least 4 m2/g at a temperature of 1,300 to 1,700° C. and a pressure of 100 to 500 kg/cm2 in vacuum for consolidation to a density of at least 95% of the theory, then HIP treatment at a temperature of 1,400 to 1,800° C. and a pressure of at least 500 kg/cm2. By these methods, allegedly, YAG sintered bodies having improved light transmission and high density are manufactured.
Furthermore, JP 2638669 (Patent Document 5) discloses a method for manufacturing a ceramic body comprising the steps of forming a green compact of appropriate shape and composition, pre-sintering the compact at a temperature range of 1,350 to 1,650° C., HIP treatment at a temperature of 1,350 to 1,700° C., and re-sintering at a temperature beyond 1,650° C. By this method, allegedly, highly transparent polycrystalline ceramic bodies are manufactured.
Besides, JP-A H06-211573 (Patent Document 6) discloses a method for manufacturing a transparent Y2O3 sintered body comprising the steps of sintering a Y2O3 powder having a purity of at least 99.8% and a primary particle average size of 0.01 to 1 μm, to a density of at least 94% of the theory, then subjecting the sintered body to HIP treatment at a temperature range of 1,600 to 2,200° C. and a gas pressure of at least 100 kg/cm2. By this method, allegedly, there are manufactured pure Y2O3 sintered bodies of the system that does not contain radioactive ThO2 as sintering aid or does not contain LiF and BeO.
Besides, JP 4237707 (Patent Document 7) discloses a rare earth garnet sintered body obtained via HIP and annealing in a pressurized oxygen atmosphere, having an average crystallite size of 0.9 to 9 μm, a light loss factor of up to 0.002 cm−1 at measurement wavelength 1.06 μm, and a transmitted wavefront strain of up to 0.05 λcm−1 at measurement wavelength 633 nm. Allegedly, there is obtained a garnet sintered body which is uncolored, reduced in light loss, and prevented from pore formation, and has a light loss factor of up to 0.002 cm−1 at measurement wavelength 1.06 μm.
Further, JP-A 2008-001556 (Patent Document 8) discloses a method for preparing a light-transmissive rare earth gallium garnet sintered body, comprising the steps of compacting a high purity rare earth oxide powder of purity at least 99.9% containing 5 ppm by weight—less than 1000 ppm by weight, calculated as metal, of at least one element selected from the group consisting of Ge, Sn, Sr and Ba as sintering aid, with a binder, into a compact having a density of at least 58% of the theoretical density, heat treating the compact to burn out the binder, firing the compact in an atmosphere of hydrogen gas, argon gas or a mixture thereof, or in vacuum, at 1,400 to 1,650° C. for at least 0.5 hour, and thereafter, effecting HIP treatment at a temperature of 1,000 to 1,650° C. and a pressure of 49 to 196 MPa. Allegedly, this method facilitates consolidation and improves light transmittance.
Furthermore, JP-A 2008-143726 (Patent Document 9) discloses a method for preparing a polycrystalline transparent Y2O3 ceramic for electron beam fluorescence in the form of a polycrystalline sintered body composed mainly of Y2O3, the polycrystalline sintered body having a porosity of up to 0.1% and an average crystal grain size of 5 to 300 μm, and containing a lanthanide element, the method comprising the steps of primarily firing a compact containing a Y2O3 powder and a lanthanide oxide powder in an oxygen atmosphere at 1,500 to 1,800° C. to form a primary fired body, and secondarily firing the primary fired body at a temperature of 1,600 to 1,800° C. and a pressure of 49 to 198 MPa. Allegedly, there is produced a polycrystalline transparent Y2O3 ceramic for electron beam fluorescence, having a mass scale productivity, containing a high concentration of fluorescent element (lanthanide element), and having the fluorescent element uniformly dispersed in the overall range of polycrystalline transparent Y2O3 ceramic.
Recently, JP-A 2010-241678 (Patent Document 10) discloses a method for preparing an optical ceramic material of the formula: A2+xByDzE7 wherein −1.15≦x≦1.1, 0≦y≦3, 0≦z≦1.6, 3x+4y+5z=8, A is at least one trivalent cation selected from rare earth ions, B is at least one tetravalent cation, D is at least one pentavalent cation, and E is at least one divalent anion, the method comprising the steps of forming a compact from a powder mixture of starting materials containing at least one sintering aid selected from the group consisting of SiO2, TiO2, ZrO2, HfO2, Al2O3 and fluorides, pre-sintering the compact at a temperature of preferably 500 to 900° C., and sintering the pre-sintered compact at a temperature of 1,400 to 1,900° C., and compressing the sintered compact in vacuum, preferably at a temperature of 1,400 to 2,000° C. and preferably a pressure of 10 to 198 MPa (HIP treatment). Allegedly, an optical ceramic material having optical properties equivalent to single crystal is produced.
As described above, active efforts are recently made on the development of oxide sintered bodies having light transmission, especially the development of oxide sintered bodies via HIP treatment step. In particular, a variety of studies are carried out to improve the light transmission of sintered bodies.